Rubber Keypad Design Guide

Thanks for considering RSPI for your rubber keypad projects.  We offer outstanding customer service through out the design and production of your keypad.  RSPI has been working with a small-dedicated network of suppliers in Asia for many years and have developed 100’s of projects during this time.  We have an understanding of the strengths of each of our suppliers and can match your project to the right supplier.  This experience with multiple suppliers allows us more control over your project and more input into production, timing, cost and quality than other sourcing agents. 

If you need additional information please contact us directly at (414) 546-4417 or email sales@membrane-switches.biz.

Website: http://www.membrane-switches.biz

Design Guide Introduction

This design guide is intended to offer basic information on rubber technology and to provide some basic design guidelines for you to follow as you consider your project.

Silicone rubber keypads are the most widely used form of switch technology today.  They offer reliability, long life and design flexibility.

Silicone Rubber Benefits 

Silicone rubber is an excellent choice for device operation controls.  It possesses the following characteristics;

• Excellent resistance to both heat and low temperature (-55° C to 250° C)        
• Minimum noise generation due to soft and elastic contact structure
• Minimum abrasion and high resistance to SO₂ and oxidation even in heavy humidity

In addition, silicone rubber offers several features that make its design and usage flexible;

• Design both tactile and linear feedback
• Translucent materials available
• Cost effective
• Multi-color designs easily accommodated
• Water and contamination resistant

We hope you find this design guide beneficial!

Dictionary of Terms Used

Basic Key Design

Key design will vary with the functional and aesthetic requirement of the application.  Many of the available key types are described below.

Basic Key Structure

There are currently several options available for improved legend life. 

• P/R Keytops – clear plastic keytops adhered to the key mat base material
• L/C Keytops – laser etched legending
• E/R Keytops – epoxy coating deposited on the top surface of the key over printing
• Ink Coating – full coverage of keypad top surface with PC ink

Design Considerations

Snap Ratio

The snap ratio of a keypad is directly linked to the tactile feel experienced by the user.  The recommended ratio for designers to maintain is 40%-60%, if dropping below this ratio the keys will lose tactile feel but have an increased life.  Loss of tactile feel means the user will not receive a ‘click’ feedback from the actuation.

Snap ratio is measured by taking the ACTUATION FORCE (F1) – CONTACT FORCE (F2).

Tactile Feedback

The membrane shape and the size of any rubber key mat can be designed to achieve almost any combination of actuation force and tactile response.  Most applications require a positive tactile feel and a long life.  With these requirements, an actuation force of 125-150 grams and an accompanying snap ratio of 40%-60% is a good recommendation.  Other combinations can be achieved by changing the contact stroke, actuation force, key shape and material hardness.  However, as a simple rule it should be remembered that the higher the force, the longer the life, but the poorer the tactile response.  RSPI works with customers to achieve the specifications required.  Always remember to specify a higher actuation force for wider or taller keys.

A common problem with rubber keypad design is ensuring that the rocking action that can be a feature of a switch design is minimized.  The following suggestions will assist in reducing this problem.

• Add stabilizing posts on base of key
• Keep key stroke as near 0.8mm as possible
• Keep web length to a minimum
• Keep web angle close to 40
• Actuation force of 80-150 grams for keys 10-15 mm high and 150-175 grams for keys 15-25mm high

Return force should also be set at 30-35 grams to ensure that keys do not stick.

Switch Life

Membrane style and the durometer of the material are the factors that effect switch life the most.  Using higher durometer silicon, increasing the actuation force, or increasing the stroke will all decrease life.

Rubber hardness can be between 30 and 70 durometer.  Typically, most keypads are built between 40 and 60 durometer.

Minimum Key Height

For any design, calculate; Keypad Base Thickness + Bezel Thickness + Stroke of Key + 0.5mm.

Contacts

The carbon pill is the most common contact because of it’s long life (>10 million actuations) and low resistance (<100W).  The pills are usually circular with diameters ranging from 1.5-10mm and thickness from 0.4-0.6mm.  Oval shaped pills are also available in a variety of sizes.

Printed carbon contacts are available in any shape however thickness is typically only 10-20 microns and resistance around 800W.

Dipped carbon contacts offer a compromise with any shape being available and contact resistance of <300W.

Printed Circuit Board Design

Rubber key mats themselves are very reliable in operation.  However, when considering a PCB design, the environment that the keypads are used in must be considered to ensure the complete switching unit is reliable.

The choice of plating for the board is probably the most critical factor with the cheaper tin/lead solder boards not being recommended.

Gold plating over nickel plating is the preferred choice for board design with a recommended layer of 30-50 microns of gold and 100-200 microns of nickel giving a contact resistance of <100W.

Nickel plating is the next best option and the most commonly used, nickel offers good reliability but is more cost effective than gold over nickel.  A plating level >200 microns is recommended for the best overall performance. 

When designing shorting pads, always attempt to get as many shorting paths as possible to increase switch reliability and ensure the pad size is never smaller than the carbon pill by a minimum of 1.25 times.

Mechanical Drawings

To assist RSPI designers, please ensure that the following information is included in your drawings.

Typical Keypad Specifications

 

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